B2.2 Organelles and Compartmentalization

• Organelles are compartmentalized subcellular structures found within a cell, which have a specific functions.
• The following cell structures are not considered organelles:

  ●Cell wall (which is an extracellular structure) ●Cytoplasm and Cytoskeleton (they do not have a compartmentalized structure)
• The development of ultracentrifugation allowed scientists to study the function of individual organelles
• Scientists prepare cells for ultracentrifugation by cell fractionation, a process which separates cell organelles while preserving their functions.
• Eukaryotic cells store their chromosomes in a nucleus.
• Genes in chromosomes are transcribed into mRNA within the nucleus.
• The mRNA leaves the nucleus to be translated into polypeptides by ribosomes.
• Prokaryotes do not have a nucleus, which means there is no separation of the processes of transcription and translation. As mRNA is being transcribed, ribosomes attach to the mRNA and translation can begin.
• Organelles provide a protective environment for specialized activities in the cell.
• Lysosomes are membrane bound organelles which contain digestive enzymes.
• The main function of lysosomes is the digestion of macromolecules within the cell. When a cell is damaged, the enzymes are released from lysosomes, resulting in the digestion of the cell.
• Phagocytosis is process by which solid materials, such as bacteria, are taken into a cell by endocytosis.
• Lysosomes fuse with the phagocytic vacuole to form a phagosome. The lysosome’s enzymes digest the bacterium.
• Mitochondria are organelles whose function is the production of ATP through aerobic respiration.
• Chloroplasts are organelles which carry out the process of photosynthesis in plants and algae.
• Photosynthesis is the production of organic compounds (such as glucose) from carbon dioxide and water, using light energy.
• The pigment chlorophyll is found within chloroplasts and absorbs light energy, which is used in photosynthesis
• The nucleus has a double membrane with pores.
• Ribosomes synthesize polypeptides (proteins) by translating mRNA. They are composed of a large ribosomal subunit and a small ribosomal subunit. Both composed of (ribosomal) rRNA and proteins.
• In eukaryotic cells, ribosomes can be floating free in the cytoplasm, or attached to the rough endoplasmic reticulum.
• The free-floating ribosomes (in eukaryotes) synthesize proteins to be used in the cell.
• The ribosomes attached to the rough endoplasmic reticulum synthesize proteins to be transported out of the cell, or for the production of lysosome enzymes.
• The Golgi apparatus is composed of a series of flattened sacs, known as cisternae. Vesicles, containing proteins from the RER, move to and fuse with the Golgi apparatus. The Golgi apparatus modifies the proteins and packages them into secretory vesicles. The secretory vesicles move towards the plasma membrane and secrete the protein by exocytosis.